Porous boron doped diamond for dopamine sensing: Effect of boron doping level on morphology and electrochemical performance

Highlights: • Planar and porous BDD films deposited at B/C ratios of 500–8000 ppm characterised. • Electrochemical behaviour of porous BDD is affected by the presence of sp² carbon. • Following increased growth time of porous layers ("thicker" BDD), behaviour resembles planar electrodes. • For dopamine, the lowest detection limit of 0.21 μmol L⁻¹ achieved on 4000 ppm "thicker" porous electrodes. • Selectivity of "thicker" porous BDD enhanced for dopamine detection in excess of AA, UA, and paracetamol. -- Abstract: Complex characterization of as-deposited conventional planar and newly available porous boron doped diamond (BDD) films with various boron doping levels (B/C ratio in the gas phase: 500 ppm, 1000 ppm, 2000 ppm, 4000 ppm, and 8000 ppm) was carried out by scanning electron microscopy, Raman spectroscopy and electrochemistry by performing cyclic voltammetric (CV) experiments with outer- and inner-sphere redox probes [Ru(NH₃)₆]^3+/2+, [Fe(CN)₆]^3−/4−, and dopamine. The double layer capacitance was estimated from CV and electrochemical impedance spectroscopic measurements. For planar BDD films, a roughly 80% increase in the double layer capacitance, a 100 mV narrower potential window and a 51 mV negative shift of the peak potential of dopamine were observed when the boron doping level increased from 500 ppm to 8000 ppm. Conversely, higher sp² carbon content confirmed by Raman spectra affects the electrochemical performance of porous samples more significantly than the doping level. For the set of porous BDD electrodes, the double layer capacitance decreased by ca 95% with increasing boron content, peak-to-peak separation values of the studied inorganic redox markers were lower than 59 mV, suggesting their partial adsorption in the bulk of the material, and lower potentials for dopamine oxidation approaching values obtained for other sp² carbon-based materials were recorded, but no dependency of potential values on boron concentration was revealed. Enhanced electron transfer rate on porous BDD materials resulted in increased sensitivity for dopamine detection using square-wave voltammetry in contrast to the planar set of electrodes. The lowest detection limit of 2 × 10⁻⁷ mol L⁻¹ was estimated in a medium of 0.1 mol L⁻¹ phosphate buffer pH 7.4 on B/C 4000 ppm porous electrodes with increased BDD growth time, without any activation applied between the individual scans. Moreover, for selected porous electrodes, improved selectivity for dopamine sensing in excess of common interfering compounds such as uric acid, ascorbic acid and paracetamol was achieved.